Coral Reefs

Structures built by animals, powered by algae.

Coral reefs are built by colonial polyps that secrete calcium carbonate skeletons. The reef is the accumulated skeleton of many generations.

The symbiosis

Reef-building corals host zooxanthellae, single-celled algae living inside their tissues. The algae photosynthesise and pass most of the sugar to the coral; the coral supplies shelter and nutrients. This is why reefs need clear, shallow, sunlit water.

Bleaching

Under heat or other stress the coral expels its algae and turns white. Bleaching is not death — a coral can recover if conditions return — but a bleached colony is starving and prolonged bleaching kills it.